In teleconferencing systems where several persons at a conference room facility are connected to a telephone network or other communication system via a single audio arrangement, reverberation and ambient noise in the room often degrade the audio signal therefrom. It is generally necessary to adapt the system so that speech and other sounds from talker locations in a room are selected for transmission while reverberation and ambient room noise are rejected. Several schemes have been developed to improve the effectiveness of conferencing communication. One type of arrangement utilizes several microphones placed on a conference table, on the conference participants, or in the ceiling of the conference room. In this way, the microphone may selectively discriminate against unwanted sounds. It has been observed, however, that the level of noise and reverberation obtained from such multiple microphone arrangements detracts from the intelligibility of the talker signals.
A vertical line array of microphones such as disclosed in U.S. Pat. No. 4,311,874 issued Jan. 19, 1982 to R. L. Wallace, Jr. and assigned to the same assignee, provides an effective transducer for large groups gathered around conference tables since its directivity pattern is toroidal in the plane of the talker's heads. Such arrays discriminate well against noise and sounds reflected from hard surfaces in a room. A similar toroidal pattern can be obtained using a plurality of acoustic pipes and a single transducer as described in the article, "Line Microphones" by Harry F. Olsen appearing in the Proceedings of the Institute of Radio Engineers, Vol. 27, pp. 438-446, July 1939. In order to provide the desired directional response pattern, however, the microphone array must be positioned in the center of the conference table. In such a position, the transducer structure may be distracting to conference participants and may obstruct their view of other participants. It is an object of the invention to provide an improved acoustic transducer structure for teleconferencing arrangements that discriminates against acoustic noise and reverberation without obstructing vision of participants.